Isoprene is a significant source of atmospheric organic aerosol; however, the oxidation pathways that lead to secondary organic aerosol (SOA) have remained elusive. Here, we identify the role of two key reactive intermediates, epoxydiols of isoprene (IEPOX ¼ β-IEPOX þ δ-IEPOX) and methacryloylperoxynitrate (MPAN), which are formed during isoprene oxidation under low-and high-NO x conditions, respectively. Isoprene low-NO x SOA is enhanced in the presence of acidified sulfate seed aerosol (mass yield 28.6%) over that in the presence of neutral aerosol (mass yield 1.3%). Increased uptake of IEPOX by acid-catalyzed particle-phase reactions is shown to explain this enhancement. Under high-NO x conditions, isoprene SOA formation occurs through oxidation of its secondgeneration product, MPAN. The similarity of the composition of SOA formed from the photooxidation of MPAN to that formed from isoprene and methacrolein demonstrates the role of MPAN in the formation of isoprene high-NO x SOA. Reactions of IEPOX and MPAN in the presence of anthropogenic pollutants (i.e., acidic aerosol produced from the oxidation of SO 2 and NO 2 , respectively) could be a substantial source of "missing urban SOA" not included in current atmospheric models.acid-catalyzed particle-phase reactions | epoxides | methacryloylperoxynitrate | organosulfates I soprene (2-methyl-1,3-butadiene, C 5 H 8 ) is the most abundant nonmethane hydrocarbon emitted into the Earth's atmosphere, with emissions estimated to be 440-660 TgC yr −1 (1). The atmospheric hydroxyl (OH) radical-initiated oxidation of isoprene, so-called photooxidation, plays a key role in establishing the balance of hydrogen oxide (HO x ¼ OH þ HO 2 ) radicals in vegetated areas (2, 3) and influences urban ozone formation in populated areas blanketed with biogenic emissions (4). Formation of low-volatility compounds during isoprene oxidation has been estimated to be the single largest source of atmospheric organic aerosol [i.e., secondary organic aerosol (SOA)] (5-8).The photooxidation of unsaturated volatile organic compounds (VOCs) proceeds through formation of a hydroxy peroxy (RO 2 ) radical, the fate of which depends on the concentration of nitrogen oxides (NO x ¼ NO þ NO 2 ). Higher SOA yields from isoprene are observed under low-NO x (or NO x -free) conditions; in this regime, RO 2 radicals react primarily with HO 2 , a pathway that tends to produce lower-volatility oxidation products than that involving the reaction of RO 2 with NO (9-11). Under high-NO x conditions, RO 2 radicals react with NO to produce alkoxy (RO) radicals, or as a minor pathway, organic nitrates (RONO 2 ). For small VOCs (≤C 10 ), like isoprene, these RO radicals generally fragment into smaller more volatile products, resulting in small amounts of SOA (9-11). Despite the fact that SOA from isoprene has been extensively studied (8), the chemical pathways to its formation under both low-and high-NO x conditions have remained unclear. In this study we examine the mechanism of isoprene SOA formation in these two ...

Epoxydiols are produced in the gas phase from the photo-oxidation of isoprene in the absence of significant mixing ratios of nitrogen oxides (NO x ). The reactive uptake of these compounds onto acidic aerosols has been shown to produce secondary organic aerosol (SOA). To better characterize the fate of isoprene epoxydiols in the aerosol phase, the kinetics and products of the acid-catalyzed ring-opening reactions of four hydroxy-substituted epoxides were studied by nuclear magnetic resonance (NMR) techniques. Polyols and sulfate esters are observed from the ring-opening of the epoxides in solutions of H2SO4/Na2SO4. Likewise, polyols and nitrate esters are produced in solutions of HNO3/NaNO3. In sulfuric acid, the rate of acid-catalyzed ring-opening is dependent on hydronium ion activity, sulfate ion, and bisulfate. The rates are much slower than the nonhydroxylated equivalent epoxides; however, the hydroxyl groups make them much more water-soluble. A model was constructed with the major channels for epoxydiol loss (i.e., aerosol-phase ring-opening, gas-phase oxidation, and deposition). In the atmosphere, SOA formation from epoxydiols will depend on a number of variables (e.g., pH and aerosol water content) with the yield of ring-opening products varying from less than 1% to greater than 50%.

Recent laboratory studies have demonstrated that isoprene oxidation products can partition to atmospheric aerosols by reacting with condensed phase sulfuric acid, forming low-volatility organosulfate compounds. We have identified organosulfate compounds in free tropospheric aerosols by single particle mass spectrometry during several airborne field campaigns. One of these organosulfates is identified as the sulfate ester of IEPOX, a second generation oxidation product of isoprene. The patterns of IEPOX sulfate ester in ambient data generally followed the aerosol acidity and NO x dependence established by laboratory studies. Detection of the IEPOX sulfate ester was most sensitive using reduced ionization laser power, when it was observed in up to 80% of particles in the tropical free troposphere. Based on laboratory mass calibrations, IEPOX added >0.4% to tropospheric aerosol mass in the remote tropics and up to 20% in regions downwind of isoprene sources. In the southeastern United States, when acidic aerosol was exposed to fresh isoprene emissions, accumulation of IEPOX increased aerosol mass by up to 3%. The IEPOX sulfate ester is therefore one of the most abundant single organic compounds measured in atmospheric aerosol. Our data show that acidity-dependent IEPOX uptake is a mechanism by which anthropogenic SO 2 and marine dimethyl sulfide emissions generate secondary biogenic aerosol mass throughout the troposphere.acid-catalyzed particle phase reactions | epoxides | free troposphere | secondary organic aerosol

Isoprene-derived epoxydiols (IEPOX) are identified in ambient aerosol samples for the first time, together with other previously identified isoprene tracers (i.e., 2-methyltetrols, 2-methylglyceric acid, C 5 -alkenetriols, and organosulfate derivatives of 2-methyltetrols). Fine ambient aerosol collected in downtown Atlanta, GA and rural Yorkville, GA during the 2008 August Mini-Intensive Gas and Aerosol Study (AMIGAS) was analyzed using both gas chromatography/quadrupole mass spectrometry (GC/MS) and gas chromatography/time-of-flight mass spectrometry (GC/TOFMS) with prior trimethylsilylation. Mass concentrations of IEPOX ranged from ∼1 to 24 ng m -3 in the aerosol collected from the two sites. Detection of particle-phase IEPOX in the AMIGAS samples supports recent laboratory results that gas-phase IEPOX produced from the photooxidation of isoprene under low-NO x conditions is a key precursor of ambient isoprene secondary organic aerosol (SOA) formation. On average, the sum of the mass concentrations of IEPOX and the measured isoprene SOA tracers accounted for about 3% of the organic carbon, demonstrating the significance of isoprene oxidation to the formation of ambient aerosol in this region.